Unimolecular dissociation dynamics of vinyl chloride on the ground potential energy surface: The method of excitation and product state distributionsof HCl and Cl fragments

The unimolecular dissociation dynamics of vinyl chloride on the ground elec tronic potential energy surface have been investigated. The vibrationally e xcited vinyl chloride in its ground electronic state is prepared using the isomerization process of alpha -chloroethylidene radical to vinyl chloride via the: hydrogen atom migration, where the chloroethylidene radical is pro duced by the ultraviolet photolysis of 3-methyl-3-chlorodiazirine. The viny l chloride molecule formed in this excitation scheme is highly vibrationall y excited in its ground electronic state due to the bond formation between two carbon atoms, and undergoes unimolecular reactions of HCl elimination o r C-Cl bond fission. The rotational and vibrational state distributions of the HCl fragments and the spin-orbit state branching ratio of the Cl atoms have been measured with a resonantly enhanced multiphoton ionization (REMPI )/time-of-flight mass spectrometry. The overall state distributions of the HCl and Cl fragments are much colder than those in the 193 nm photodissocia tion. The rotational distributions of the HCl(nu = 0) and HCl(nu = 1) fragm ents Fit to the Boltzmann distributions at T-rot = 470 and 130 K, respectiv ely. The vibrational branching ratio of HCl(nu = 1)/HCl(nu = 0) and the spi n--orbit state branching ratio of Cl*(P-2(1/2))/Cl(2P(3/2)) are measured to be 0.15 +/- 0.03 and 0.15 +/- 0.02, respectively. The differences in the d ynamical observations of vinychloride produced by this excitation scheme an d the 193 nm photoexcitation are interpreted in terms of the large differen ce of excitation energies between two excitation schemes.